1. Field of the Invention
The invention relates to a heat treatment device of the light irradiation type in which a semiconductor wafer (hereinafter called a wafer) is subjected to rapid heating, holding at a high temperature, and rapid cooling for layer formation, diffusion, baking or for similar purposes. Furthermore, the invention relates to a heat treatment process of the light irradiation type.
2. Description of Related Art
Heat treatment of the light irradiation type in the production of semiconductors, like layer formation, diffusion, baking or the like, is carried out in a wide range. In each of these treatments, a wafer is heated to a high temperature.
If a heat treatment device of the light irradiation type is used for this heat treatment, the wafer can be quickly heated and its temperature increased within a few seconds to a few dozen seconds to a temperature of at least 1000xc2x0 C. When the light irradiation is stopped, it can be quickly cooled.
If, however, when the wafer is heated, the temperature distribution in the wafer ceases to be uniform, a phenomenon called slip occurs in the wafer, i.e., a dislocation defect. In this case, there is the danger that scrap will result. Therefore, in the heat treatment of the wafer using a heat treatment device of the light irradiation type it is necessary to carry out heating, holding at a high temperature, and cooling such that the temperature distribution of the wafer becomes uniform.
For example, in the case in which a wafer is heated to 1050xc2x0 C., it is possible for the above described slip to form if the temperature difference within the wafer surface is 2xc2x0 C. or more. To suppress formation of slip, it is desirable to keep the temperature difference within the wafer surface within 1xc2x0 C.
Furthermore, when the wafer is heated for layer formation, the wafer must be heated with very precise uniformity within the surface in order to form a layer with a uniform thickness.
If the wafer is kept at a high temperature in the heat treatment device of the light irradiation type, the temperature of the peripheral area of the wafer becomes lower even if the entire surface of the wafer is irradiated with uniform irradiance. For example, a wafer is generally subjected to oxidation treatment by its being heated to roughly 1100xc2x0 C. However, in the case in which the temperature of the middle area of the wafer is 1100xc2x0 C., the temperature of the peripheral area becomes about 30xc2x0 C. less than in the middle area; this causes the above described slip.
The reason for lowering the temperature of the peripheral area of the wafer is that heat is emitted from the side surface of the wafer. The heat emission from the side surface causes heat flow in the wafer which results in a temperature distribution. To prevent this, a process was proposed long ago in which an auxiliary material with the same heat capacity as that of the wafer is placed such that the outside periphery of the wafer is surrounded by it. This auxiliary material is generally called a guard ring.
When measures are taken so that the wafer and the guard ring can be considered a single, integral plate body, the temperature of the peripheral area of the wafer is prevented from decreasing, because the peripheral area of the wafer does not represent the peripheral area of the above described plate body, when both the wafer and also the guard ring are heated by uniform light irradiation.
Since the guard ring is arranged such that it surrounds the outer circumference of the wafer, it can also be used at the same time as the wafer holding material if it is provided with a device which holds the peripheral edge area of the wafer securely. Therefore, there are many cases in which the guard ring also has the function of holding the wafer securely.
A guard ring is specifically a component which balances the temperature drop as a result of heat emission from a side surface of the wafer or its vicinity and makes the temperature of the wafer uniform. There are also many cases in which the guard ring is used as the wafer holding material.
But it is difficult in practice to produce the guard ring such that it can be regarded as integral with the wafer (i.e., the heat capacity is made the same). The reason for this is described below.
(1) It is very difficult to produce the guard ring from the same material as the wafer. If the guard ring is produced from the same material as the wafer, i.e., from silicon (Si), the heat capacity of the wafer and the guard ring can be made the same. However, it is very difficult to process silicon into a form in which the wafer can be held securely. Furthermore, silicon is deformed when it is repeatedly exposed to a great temperature difference, by which the function as a guard ring is lost.
(2) As a material which can be processed relatively easily and which has a somewhat greater heat capacity than silicon, but a close value, there is silicon carbide (SiC). Generally silicon carbide is used for the guard ring. But, since the thickness of the silicon carbide cannot be less than 1 mm due to a problem (yield) with respect to processing, the thickness is greater than the wafer thickness of 0.7 to 0.8 mm.
(3) Due to the difference in the specific heat between silicon and silicon carbide and the difference in the thickness between the two, the heat capacity of the guard ring per unit of area is roughly 1.5 times as high as in a wafer when heated to a high temperature. Therefore, it is necessary to heat the guard ring with a greater power than the wafer in order to eliminate the above described difference of heat capacity between the wafer and the guard ring.
FIG. 11 shows a conventional example of the arrangement of a heat treatment device of the light irradiation type. The figure shows the cross-sectional arrangement in which the heat treatment device of the light irradiation type was cut in a surface which passes through the middle of the device and intersects the wafer surface orthogonally. As is shown in FIG. 11, in a light source part 1, there are several (in this example, nine) concentrically arranged circular filament lamps L2 to L10 with different ring diameters but with the same pitch (20 mm). The ring diameter of the innermost lamp L2 is 50 mm. In order to irradiate its inner side, a lamp L1 is installed which is hermetically sealed at one end. The input power of the circular lamps L2 to L10 is, for example, 240 W/cm.
Behind the lamps L2 to L10, there is a concave-shaped mirror 1a which reflects the light from the lamps L2 to L10 onto the side of the wafer W. The light from the light source part 1 is reflected by the lamps L1 to L10 directly or by the mirror 1a and is emitted via a quartz window 2 (in this example, with a thickness of 20 mm) onto the wafer which is placed on the guard ring 3. The distance between the lamps L1 to L10 and the wafer W, in this example, is 50 mm. The guard ring 3 is held securely by a holding member 12 which is located in a chamber 11, and at the same time, has the function of the wafer holding material.
As shown in FIG. 11, in the light source part 1, there are not only the lamps L1 to L8 which irradiate the area in which the wafer W is present, but also lamps L9 and L10 which are used for broadening the light irradiation area. The guard ring 3 is also heated by light irradiation. The guard ring 3 is produced by the above described processing of silicon carbide and has a heat capacity roughly 1.5 times as high as that of the wafer, as was described above. Therefore, the input power of the lamps L9 and L10 has been increased so that their output power is increased, and the attempt is made to emit a large amount of radiant energy onto the guard ring 3 in order to eliminate the above described difference of heat capacity.
The light from the lamps L9 and L10 for broadening of the above described light irradiation area is, however, emitted broadened. Therefore, it is emitted not only onto the guard ring 3, but also onto the wafer W. When the output power of the lamps L9 and L10 is increased and thus radiated, therefore the temperature of the wafer W increases, by which heating with a uniform temperature becomes difficult.
Furthermore, even if the output power of the lamps L9 and L10 is increased, it is difficult to supply radiant energy to the guard ring with high efficiency which is sufficient for eliminating the difference of heat capacity between the wafer W and the guard ring 3 because the emitted light broadens. The temperature of the guard ring 3 is therefore lowered in a wide area in which the wafer W is in contact with the guard ring 3 in a discontinuous way; this causes slip.
If the output power of the lamps L9 and L10 is increased, the irradiance is also increased in the area of the guard ring 3. However, since the wafer W is also irradiated, the irradiance in the wafer edge area is increased. The temperature of the wafer W is therefore increased in the direction toward its edge area. On the other hand, the light from the lamps L9 and L10 is broadened, by which the guard ring 3 cannot acquire any energy which balances the above described difference of heat capacity. The temperature is therefore lowered in the area in which the wafer W is in contact with the guard ring 3 in a discontinuous manner; in this way, slips occurs in the peripheral area in which the temperature is not uniform.
To make available the energy which is used to balance the difference of the heat capacity between the wafer W and the guard ring 3, the input power for the lamps L9 and L10 must be increased even more; this is not effective.
The invention was devised to eliminate the above described defect in the prior art. The primary object of the invention is to enable uniform heating of the wafer, and moreover, to heat the guard ring with high efficiency by focusing the light from the lamps for heating the guard ring only on the guard ring.
The object is achieved as follows:
(1) In a heat treatment device of the light irradiation type which comprises:
a light source part in which there are several concentrically arranged circular filament lamps with different ring diameters, and in which there is a mirror behind the above described filament lamps;
a wafer located on the side opposite the mirror; and
a guard ring which is located in the peripheral area of the wafer in that the above described several filament lamps of the light source part are comprised of a first lamp group located opposite the wafer and of a second lamp group located opposite the guard ring on the outer periphery of the first lamp group so that the light emitted from the second lamp group is not radiated onto the wafer, that furthermore the distance between the lamps in the second lamp group and the guard ring in the direction of light irradiation (in the direction perpendicular to the wafer surface) is made larger than the distance between the lamps of the first lamp group and the wafer in the direction of light irradiation (in the direction perpendicular to the wafer surface), and furthermore, that the side wall formed between the second lamp group and the first lamp group has a mirror surface by which the light which has been emitted from the lamps of the second lamp group and which faces toward the wafer is reflected in the direction toward the guard ring.
This prevents the light from being emitted from the lamps of the second lamp group onto the wafer. The light corresponding to this amount can be focused on the guard ring.
(2) In the above described approach (1), on the outermost outer periphery of the second lamp group, a second side wall is formed which extends in the direction which opens with respect to the direction of light irradiation, and which reflects the light emitted from the second lamp group in the direction toward the above described guard ring.
This means that a second side wall is formed which is provided in a direction with an angle which opens with respect to the direction in which the outer periphery of the second lamp group is irradiated with light. This side wall is used as the reflection surface. In this way, the light which is emitted from the lamps of the second lamp group in the direction toward the outside of the guard ring can be focused on the guard ring and the radiant energy emitted onto the guard ring can be increased.
(3) In the above described approaches (1) and (2), on the outside periphery of the guard ring there is a second mirror which reflects the light emitted from the second lamp group in the direction to the guard ring.
This means that, on the outer periphery of the guard ring, there is a second mirror which in one direction is provided with an angle which opens with respect to the direction of the light source part. In this way, the light emitted onto the outer side of the guard ring can be focused on the guard ring and thus, the radiant energy emitted onto the guard ring can be increased.